This invention relates to cable termination systems, more particularly to undersea cable (or umbilical) termination systems.
Modern sub-sea cables are often laid in hazardous environments, and can be subject to high force loadings. In order to ruggedize the cables so that they are capable of withstanding these forces, they usually comprise a woven outer armour layer. For heavyweight cables, this armour layer may be thick steel cables, wires or similar. This sturdy armoured layer preferably takes the strain when the cable is stretched or bent, protecting the inner components of the cable. However, the cables are consequently large and heavy, and are not very flexible which can make working with them difficult.
In the art, the term cable typically refers to structures having cores and fibre optics. An umbilical generally refers to a structure that may include cables as well as hoses, fibre optics etc. In this document, references to cables refer to any kind of cable structures including umbilical systems.
In some applications, it is desirable to use a lighter cable, and one that is less resistant to bending. Such lightweight cables may have a lightweight armour layer comprising synthetic fibres. Such an armour layer is capable of withstanding tension without impairing the flexibility of the cable. Another term used in the art for the individual strands or bundles thereof in such an armour layer is “strength member”. This is because the main function served by the synthetic fibres is adding tensile strength and resilience to the cable, rather than armouring it against external influences.
The part of a cable close to its termination, for example, at the cable's final destination, is often subject to a particularly high strain as the flexible, moveable cable is connected to a stationary, rigid object. As such, it is preferable that the cable is securely coupled to the structure the cable is connected to, otherwise the termination forms a natural weak point in the cable system. This is carried out through a cable termination structure, which is mounted on the cable and coupled to the structure. Cable termination structures are generally securely coupled to the armour layer or strength members of a cable for transferring forces to them, leaving the inner components of the cable to pass through or around them.
An example of a cable termination structure is illustrated in FIG. 1. The key component of this structure is the spike, 100, which is in the form of a hollow cone. The hollow in the centre of the spike allows the central components 104 of the cable through, and the strength members 103 pass around the outside of the spike and are held in place after passing around the wide end 101 of the spike. Such a termination structure will normally be encased in resin in order to hold the armour layer in place.
One of the problems with this cable termination structure is that the strength members are laid unevenly along the spike. Those at the tip 102 of the spike are closer and tighter packed than those at end 101 of the spike. This results in un-even contact pressure over the whole surface of the spike, which is undesirable. A further problem with this termination structure is that the tip of the spike 102 should have a small radius of curvature, in order to protect the strength members as they exit the cable and run over the spike. Truncating the spike tip does not alleviate the situation, and further induces higher side load stresses in the strength members when the cable is subjected to a torque.
A further problem with this cable termination structure emerges at high loads. Those strength members at the tip 102 of the spike, due to their being tightly packed, may actually crush and sever each other, where they press on each other. This is obviously undesirable as transient loads are capable of destroying the integrity of the termination. Furthermore, over time the resins used to retain the strength members at the tip 102 of the spike can migrate to the wide end 101 of the spike, where the high tensions can break down the resin. This produces sharp crystal particles that can sever the strength members in this critical load-bearing zone.